I can understand your concern regarding these negative tests.
However, only a careful systematic approach will ensure that nothing is missed out.
Presently, peripheral blood and bone marrow
cell morphology need to be done, if they've not been done already. The platelets and their bone marrow precursors need to be carefully assessed, so as to understand what exactly is responsible for the low counts. Since you said the count has been so since birth, we may be dealing with a congenital platelet disorder, but the tests you mentioned are insufficient to unravel which one it is.
You should see a specialist hematologist
, to help you through the process.